The present invention relates generally to reflective articles. The invention has particular application to retroreflective sheeting with a multiplicity of cube corner elements.
Retroreflective materials are configured to receive light rays impinging upon a viewing surface and so alter the rays that they are reflected back toward their sources. Retroreflective material is generally used to enhance low-light visibility of articles to which the retroreflective material is attached. Such material is used in a variety of applications ranging from traffic signs to bicycle reflectors. By enhancing low-light visibility, retroreflective materials enhance safety, provide decoration, and increase conspicuity in general.
Two known types of retroreflective material include microsphere-based sheeting and cube corner sheeting. Microsphere-based sheeting, sometimes referred to as "beaded" sheeting, employs a multitude of microspheres typically at least partially imbedded in a binder layer and having associated specular or diffuse reflecting materials (e.g., pigment particles, metal flakes, vapor coats) to retroreflect incident light. In general, however, such sheeting has a lower retroreflective efficiency than cube corner sheeting.
Cube corner retroreflective sheeting comprises a body portion typically having a substantially planar viewing surface and a structured surface comprising a plurality of cube corner elements. Each cube corner element comprises three approximately mutually perpendicular optical faces that intersect at a cube apex or, where the cube apex is truncated, that otherwise converge at an uppermost portion. It is known to treat the structured surface with a specularly reflective coating to improve performance at high entrance angles. An example of this is vapor-coated retroreflective sheeting.
Cube corner sheeting typically has a much higher retroreflectance than beaded sheeting, where retroreflectance is expressed in units of candelas per lux per square meter. However, certain graphics applications require not only high retroreflectance but high daytime "whiteness". The whiteness of an object is sometimes described in terms of the second of the tristimulus coordinates (X,Y,Z) for the object, and thus is referred to as "cap-Y". The cap-Y scale ranges from 0 for a perfectly black object to 100 for a perfectly white object. The whiteness of an object is also sometimes described in terms of its "Luminance Factor", ranging from 0 to 1. If the daytime whiteness of cube corner sheeting could be increased, without substantially reducing retroreflectance, such sheeting could find broader application in graphics applications. Cube corner sheetings which have an aluminum or other metal vapor coat applied to the structured surface tend to have a somewhat grayish appearance, rather than white.
One way that the whiteness of cube corner sheeting has been increased in the past is by printing white ink on the sheeting. Such printing methods have included printing on the outside of the transparent overlay layer or printing on the structured surface of the cube layer. Although these methods have helped increase the whiteness of vapor coated cube corner sheeting, they tend to reduce or sacrifice the retroreflectiveness of the sheeting. Accordingly, the art seeks more durable, less expensive, easier to manufacture alternatives while reducing the sacrifice in retroreflectiveness.